The typical semiconductor diode laser includes an n-type layer, a p-type layer and an undoped active layer between them such that when the diode is forward biased electrons and holes recombine in the active region layer with the resulting emission of light. The layers adjacent to the active layer typically have a lower index of refraction than the active layer and form cladding layers that confine the emitted light to the active layer and sometimes to adjacent layers. In fabricating such devices each individual semiconductor layer is grown epitaxially using a crystal growth technique.
One of the best known crystal growth techniques is low pressure metallo-organic chemical vapor deposition (LP-MOCVD) carried on in a controlled atmosphere reactor oven. In this method, source gases such as metal alkyls and hydrides are mixed and pyrolized in a hydrogen atmosphere to grow thin single crystals of semiconductor material upon a substrate. Specifically, a first growth stage is initiated when a first gaseous mixture is introduced into the reactor and completed when a thin single crystal layer is grown. After the first growth stage, a brief interval may follow to allow the gases from the first growth stage to clear out of the reactor and to ready the second gaseous mixture. Then, the second growth stage is initiated by introducing the second gaseous mixture into the reactor for growth of the next thin crystal layer. The growth stages are repeated in this manner until the desired heterostructure is grown.
It has recently become desirable to produce 8xx nm lasers whose quantum well and waveguide layers are aluminum free with some aluminum content permitted only in the cladding layers. Devices having aluminum-free quantum well and waveguide layers are desirable because they are not susceptible to COD (catastrophic optical destruction) which may occur in lasers containing aluminum in these layers. Although high output powers have been obtained from the AlGa(In)As active-layer devices, long-term reliability is still an open question because, even if the mirror facets are passivated, defects in the mirror facet lead to COD. Accordingly, among those heterostructures that are of interest are those which may be characterized in shorthand form as AlGaInP/GaInP/InxGa1-xAs1-yPy, where x, y are small and where the underlined portion identifies the composition of the quantum well.
Semiconductor structures for 8xx nm lasers on GaAs substrates may use various mixture proportions of Ga, In, As and P in the active region. A phase diagram for these materials is shown in FIG. 4 where the upper left-hand corner represents maximum GaAs content; the upper right-hand corner represents maximum In content; the lower left-hand corner represents maximum GaP content; and the lower right-hand corner represents maximum InP content. The straight slant line is the locus of lattice matching of the active region with the GaAs substrate while the curved lines represent the locii of mixtures resulting in layers over the range from 2.0 to 1.2 electron-volts, corresponding to emissions at wavelengths from 600 to 1000 nm.
In theory, a laser device emitting at a desired wavelength may be produced using any of the combinations of GaAs, InAs, InP and GaP falling along the corresponding electron-volt line. In practice, however, there are some limitations or tradeoffs. First, the stress increases with increasing distance from the GaAs-matching line, limiting their thickness (critical thickness).
Secondly there is a zone in which combinations of these materials are poorly miscible (miscibility gap). It is somehow desirable to avoid combinations within the zone of poor miscibility. Taking both restrictions in account in the design of 8xx nm material, the desired quantum well material becomes InxGa1-xAs1-yPy (where x, y are small), sandwiched between Ga0.5In0.5P barriers, with high Indium amount (e.g., about 50%). Therefore the task is to grow very thin low Indium containing material on top of high Indium containing material. This was studied in the last decade.
Guimaraes, et al as reported in the 1992 Journal of Crystal Growth, studied the growth of GaInP/GaAs hetero structures in LP-MOCVD reactors. Guimaraes study attempted to account for the existence of an anomalous (“Q-line”). A photoluminescent emission band of high intensity radiation occurred at the GaInP-to-GaAs interface at an energy level below the band gap of GaAs. Variations of growth temperature, AsH3 partial pressure, gas switching sequence and growth interruption times were experimented with but could not eliminate the anomalous radiation.
From the results of experiments carried out in a horizontal MOVPE reactor, Guimaraes postulated that a spurious quaternary (GaInPAs) layer was formed at the GaInP-to-GaAs interface, apparently due to the instability of the GaInP surface under AsH3 purging, which caused partial substitution of P by As (i.e., P desorption). The 1992 article reported that the formation of the quaternary layers responsible for the anomalous radiation could be prevented by introducing a 1-nm thick layer of GaP during the growth interruption. PH3 was switched off for 2 seconds and AsH3 is switched on for 3 seconds before the growth of the GaAs quantum well is started). This technique was reported to prevent “As incorporation into the GaInP layer”.
Two years later, in 1994, another group of researchers came to a different conclusion for the source and for the remediation of the anomalous radiation. Tsai, et alia reporting in the Journal of Crystal Growth concluded that the parasitic quaternary layer occurring when GaAs was grown on GaInP was caused by parasitic incorporation of unwanted indium. Tsai reported no success in suppressing the “Q-line” radiation using the 1992 Guimaraes technique, even though GaP intermediate layers of 1, 2 and 3 nm thicknesses were tried. Instead, Tsai et al resorted to using a very thin layer of AlGaAs with somewhat mixed results.
Both the Guimaraes and Tsai experiments were carried out in small-scale laboratory environments where throughput rate and product uniformity are not of primary concern. An industrial environment places great emphasis not only on these requirements but also on the ability to produce lasers having uniform and specifiable radiation wavelengths. Accordingly, it would be advantageous to provide an efficient and reliable method for producing 8xx laser crystal structures in large, multi-wafer planetary MOCVD reactors which emit at a specifiable and uniform wavelength, which do not emit spurious Q-line radiation.